atmosphere
Article
Characteristics of Preliminary Breakdown Pulses in
Positive Ground Flashes during Summer
Thunderstorms in Sweden
Dalina Johari 1,2, *, Vernon Cooray 1 , Mahbubur Rahman 1 , Pasan Hettiarachchi 1
and Mohd Muzafar Ismail 1,3
1
2
3
*
Ångström Laboratory, Division of Electricity, Department of Engineering Sciences, Uppsala University,
Box 534, 75121 Uppsala, Sweden; [email protected] (V.C.);
[email protected] (M.R.); [email protected] (P.H.);
[email protected] (M.M.I.)
Faculty of Electrical Engineering, Centre for Power Electrical Engineering Studies,
Universiti Teknologi Mara, 40450 Shah Alam, Selangor, Malaysia
Faculty of Electronics and Computer Engineering, Telecommunication Engineering Department,
Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Malacca, Malaysia
Correspondence: [email protected]; Tel.: +46-18-471-5908
Academic Editor: Farhad Rachidi
Received: 2 February 2016; Accepted: 3 March 2016; Published: 10 March 2016
Abstract: This paper presents the characteristics of the preliminary breakdown pulses (PBP)
in 51 positive ground flashes recorded during 2014 summer thunderstorms in Sweden. Electric
field measurements were conducted remotely using a broadband antenna system (up to 100 MHz)
for a recording length of 1 s with 200 ms trigger time. In the analysis, PBP trains were observed
in 86% of the cases. Based on the number of trains preceding the first return stroke, the PBP were
classified into single and multiple train PBP. Characteristics of the first PBP train were determined
and based on the initial polarity of the pulses, three types of PBP were identified. Characteristics of
the subsequent PBP trains in the multiple train PBP were also analyzed and they were compared with
the first PBP train. Based on the conceptual charge cloud configuration, we found that the inverted
dipole is consistent with our observation. We also found that PBP in positive ground flashes during
summer thunderstorms in Sweden are weak since the average ratios of the PBP peak to the first
return stroke peak lie only between 0.21 and 0.26. Possible reasons for no detection of PBP and the
different types of PBP observed were also discussed.
Keywords: lightning; positive ground flashes; preliminary breakdown pulses; multiple train PBP
1. Introduction
1.1. Positive Ground Flash
Positive ground flashes are ground flashes that transport positive charges from cloud to ground.
Although they account for only 10% of total global cloud-to-ground (CG) lightning [1], they tend to
produce strokes with higher peak current and larger charge transfers, making them more dangerous
than their negative counterpart. Among the characteristics that are thought to be associated with
positive ground flashes is the significant, in-cloud discharge activity that precedes the return stroke
(RS) lasting on average, in excess of 100 ms and 200 ms [1]. According to [2], this initial breakdown,
often referred to as preliminary breakdown, is the in-cloud (IC) process that initiates or leads to
the initiation of the downward-moving stepped leaders. The downward-moving leaders will then
propagate towards the ground, inducing upward-connecting leaders in the process that will attempt
Atmosphere 2016, 7, 39; doi:10.3390/atmos7030039
www.mdpi.com/journal/atmosphere
Atmosphere 2016, 7, 39
2 of 18
to connect to the downward-moving leaders. A complete conducting path will be formed when they
meet and a wave of ground potential will travel along the path towards the cloud in a process called
the return stroke [3].
1.2. Review on Previous Studies
As a process that initiates lightning flashes, various studies have been conducted to characterize
preliminary breakdown (PB) in both ground and cloud flashes. Observations have been made by
using electromagnetic field measurements for positive ground flashes (e.g., [4–10]), negative ground
flashes (e.g., [11–17]) and cloud flashes (e.g., [18–21]). Recently, many new findings have been made
by observations with high-speed video camera (e.g., [22–24]) and three-dimensional (3D) location
network (e.g., [25–27]). Stolzenburg et al. [22], for example, observed burst of luminosity during the
initial breakdown stage of negative ground flashes and IC flashes. They found that the increase in
luminosity in the high-speed video data is coincident with the PB pulses in the fast electric field change
records. Further study by Stolzenburg et al. [23] later showed that the burst of luminosity is directly
related to the PB pulse amplitude. In another study, Campos and Saba [24] observed one stepped
leader for a negative ground flash. Their results suggested that the beginning of the initial breakdown
pulse trains occurred simultaneously with an early, fast, and bright leader or streamer propagation.
The observed pulses were found to change from bipolar to unipolar structure, and the transition
coincided with channel luminosity reduction and propagation speed. Recently, Wu et al. [27] who
studied the PB process in IC flashes found that PB in most IC flashes has an initiation altitude that
ranges from 5–10 km and IC flashes initiated at higher altitude have lower vertical speed. They also
demonstrated that characteristics of the PB pulse trains have close connections with the initiation
altitude and upward propagation speed.
Compared to negative ground flashes, however, information about PB in positive CG lightning,
is still scarce, as pointed out by Marshal et al. in [17]. Based on the information found in the
literature, characteristics of the preliminary breakdown pulses (PBP) in positive ground flashes can be
summarized as follows.
1.2.1. Overall Pulse Shape and Polarity of the Initial Half Cycle
Individual preliminary breakdown pulses are typically bipolar as reported by many
investigators [4–9]. The initial polarity of the pulses can either be the same or opposite to that
of the first return stroke [4–10]. In contrast, polarity of the PBP in negative ground flashes was found
to be similar to the polarity of the first return stroke [8,9,13,28]. Recently, Baharudin et al. [15] observed
that some PBP in negative ground flashes can also have opposite initial polarity to that of the following
negative return strokes though the authors said it remains an open question.
Based on the pulses initial polarity, the PBP have been classified into either same or opposite
polarity PBP [4,7,8,10]. Several researchers have also identified PBP with composite initial
polarity [5,6,9] and PBP with two distinct regions [5]. Numerous reasons and hypotheses have been
suggested for the occurrence. According to Qie et al. [9], since positive ground flashes seemed to
occur when the thunderstorm started to dissipate and the main negative charge centre was weakened,
negative charge centre under the main positive charge centre may trigger same polarity PBP and
eventually form a positive ground flash. Also, they discussed that different polarity PBP trains might
take place between the main negative and lower positive charge centres (as [29] assumed), similar to
the preliminary breakdown processes in negative ground flashes, but the negative charge is largely
expended in the positive ground flashes cases and repolarized in the main positive charge centre,
initiating a positive leader to ground. As for the irregular type PBP, Qie et al. [9] stated that the PBP
trains might contain breakdown discharges between both the upper and lower two charge layers.
Gomes and Cooray [5], who identified four types of bipolar pulse trains gave two possible reasons for
the different types. One being the geometrical alignment of the channel, while the second possibility
Atmosphere 2016, 7, 39
3 of 18
is the breakdown between several combinations of regions in the cloud that may give rise to the
ground flash.
1.2.2. PBP Peak Amplitude Relative to Return Stroke Peak
Several researchers have analyzed the strength of the PBP by comparing the PBP peak amplitude
relative to the first return stroke peak. Wu et al. [8] found that positive ground flashes are unlikely to
be associated with very strong PBP and they are much weaker than those in negative ground flashes.
According to Wu et al. [8], their conclusion was in agreement with the observation by Brook et al. [12]
who noted that in winter thunderstorm, negative ground flashes could have very intense PBP that was
sometimes even stronger than the return stroke, but positive ground flashes had very weak PBP both
in winter and summer. Qie et al. [9], however, concluded that based on their results, the preliminary
breakdown process radiates strongly in positive ground flashes.
1.2.3. Temporal Characteristics
Temporal characteristics of the PBP such as the interpulse duration, overall pulse duration, and
pulse train duration have also been studied. Ushio et al. [4] found that the values for both the interpulse
duration and overall pulse duration were half of those of negative ground flashes reported by [18]
while the value of the overall pulse duration was consistent with the value of the pulses associated
with cloud flashes reported by [30]. Gomes and Cooray [5], on the other hand, found that the PBP
pulse characteristics were somewhat similar to the breakdown pulses preceding negative ground
flashes, though confined to a much wider range. Wu et al. [8] too found that some characteristics such
as the zero crossing time and pulse train duration were similar to that of negative ground flashes but
some characteristics such as the interpulse duration were different.
1.3. Significance of Study
As stated by Wu et al. [8], it is still not clear exactly what happens during preliminary breakdown
process and according to [31], it has not been established yet whether positive ground flashes draw on
the upper positive charge or the lower positive charge, or still another positive charge reservoir. Many
suggestions have been put forward but according to Qie et al. [9], the consensus about the initiating
mechanisms of positive ground flashes has not been reached. The study of the preliminary breakdown
process before positive return strokes is therefore important as it helps to resolve the understanding
of the in-cloud process that initiates positive ground flashes. Secondly, Marshal et al. [17] found that
the study of PBP in positive ground flashes is relatively limited compared to PBP in negative ground
flashes. They came to this conclusion when analyzing the percentage of lightning flashes that begin
with initial breakdown pulses. They found that only a few have investigated PBP before positive
ground flashes (e.g., [4,5]) and these studies did not include the statistics on how many positive ground
flashes begin with initial breakdown pulses. This caused them to restrict their analysis to negative
ground flashes only. The findings from this study would then add to the existing statistics of the PBP
characteristics in positive ground flashes and help to improve our overall understanding of positive
ground flashes.
1.4. Objective
This paper presents the characteristics of the PBP observed in positive ground flashes during 2014
summer thunderstorms in Uppsala, Sweden (59.837˝ N, 17.646˝ E). Electric field changes preceding
positive return strokes from remote electric field measurement were analyzed. We obtained the
percentage of the positive ground flashes with detectable PBP and the detection percentage according
to the distance of the first return stroke, and classified the PBP based on the number of trains observed
preceding the first return stroke. Their characteristics such as the PBP polarity, PBP peak amplitude
relative to the first return stroke peak and their temporal characteristics were also determined.
Comparison studies were then made with findings from previous studies.
Atmosphere 2016, 7, 39
4 of 18
1.5. Sign Convention
The atmospheric electricity sign convention is used throughout the paper according to which a
downward directed electric field is considered to be positive.
2. Experiments
The overall cloud-to-ground lightning flash is known to be composed of a number of processes
such as preliminary breakdown, stepped leaders, connecting leaders, return stroke, recoil leaders,
K-changes, continuing currents, M-components, dart leaders, and subsequent return strokes [3]. Since
these processes are associated with the motion of charge, they can be studied via measurement of the
electromagnetic fields associated with that charge motion [2].
2.1. Measurement Setup
Measurements of the electric fields generated by the ground flashes were carried out in Uppsala,
Sweden (59.837˝ N, 17.646˝ E) from June to August 2014 during the summer season. They were
conducted at a station close to the Division of Electricity, Uppsala University.
The measurement setup consisted of broadband antenna systems (comprised of a parallel plate
antenna and a vertical whip antenna), a DL850 Yokogawa transient recorder and a Meinberg M400
GPS antenna system. Signals from both antennas were fed into the transient recorder by using RG58
coaxial cables with proper termination to avoid unnecessary reflections.
The parallel plate antenna was used to detect the fast variation (i.e., fast field) of the electric field
generated by the ground flash while the vertical whip antenna was used to detect the slow variation
(i.e., slow field). Each antenna has a buffer amplifier circuit with decay time constant of 15 ms for
fast field and 1 s for slow field. Detailed description on the antenna system and the buffer electronic
circuits can be found in [32]. Further readings can also be found in [15,16] since the antenna systems
used were identical.
The transient recorder is a high-speed, high-resolution recorder with a 12-bit resolution and
sampling rate of 100 M sample/sec at 10 ns interval. The transient recorder was triggered based
on the voltage amplitude of the incoming signal from the parallel plate antenna. The trigger level
was set above noise level at 50 mV minimum. During the measurement period, the trigger level
would be changed depending on how close the thunderstorm was, monitored through the Swedish
Meteorological and Hydrological Institute (SMHI) website and observation at site. The closer the
thunderstorm to the station, the higher the trigger level would be set in order to ensure close flashes
would be recorded. The recording length was set to 1 s to ensure that the whole activity would be
captured. The transient recorder was set to work in a pre-trigger mode of 20% (i.e., 200 ms) so that
signals before and after the trigger pulse could be recorded.
The GPS antenna system provided timing information for the event. To obtain the location of
these ground flashes, data from the Swedish lightning locating system (LLS) were used. The LLS
data were also used to double-check the stroke polarity of the recorded waveforms. The Swedish LLS
consists of 10 sensors [33] with an overall detection efficiency of about 85% [34].
2.2. Sample Size
A total of 3025 lightning data were recorded during the measurement period. From this,
299 positive ground flashes were identified. After undergoing a careful selection process, a total
of 51 positive ground flashes located 6–130 km from the measuring station were selected for the final
analysis. Many signals were removed because they were too small to be analyzed while some were
removed due to signal saturation. A few were discarded because the electric field waveforms were not
fully captured by the transient recorder. Pulse activity at the end of the 1 s recording length could still
be seen which might have indicated that the process was still continuing.
Atmosphere 2016, 7, 39
5 of 18
2.3. Criteria for PBP Identification
The signatures of the electromagnetic fields depend on the physical processes of the lightning
flash, and hence, they can be used to identify the different processes associated with the ground flash.
The positive return stroke, for example, is indicated by an abrupt and considerable electric field change
pulse in the fast field record with coincident field change in the slow field record [35]. For preliminary
breakdown, the electrostatic field generated by the process starts to increase slowly for several tens to
several hundreds of milliseconds and ends with a burst of pulses having a duration of the order of a
millisecond [3]. This electrostatic field change can be seen from the slow field record while the pulse
burst, which is mainly radiation, can be seen from the fast field record.
From the measured fast field records, the PBP can be identified as the initial pulse train that
precedes the first return stroke, normally found at the beginning of the measured waveform. Once
the pulse train has been identified, we used the same criteria for the selection of individual pulses
as described in [14]. Only pulses with peak-to-peak amplitudes equal to or exceeding twice that of
the average noise level were considered since this condition would result in readable pulses for the
analysis. In addition, the pulses were considered as not belonging to the pulse train if they were
separated from the last pulse of the train by at least 2 ms. As for the noise level, it was obtained
by calculating the average value of the noise peaks at the quiet stage with little associated lightning
pulse [6].
3. Results
In the study, electric field changes preceding the first return stroke in 51 positive ground flashes
were analyzed. The analyses were done based on the radiation field pulses associated with the
preliminary breakdown process in positive ground flashes.
3.1. Positive Ground Flashes with Detectable PBP Trains
Out of 51 positive ground flashes (+CG) selected, PBP trains were observed in 44 (86%)
of the cases while for the remaining 7 (14%), no PBP trains were detected. In previous studies,
Schumann et al. [7] found that all flashes (100%) presented breakdown pulses prior to the return
stroke while Qie et al. [9] found 48% of the positive ground flashes were preceded by PBP. In contrast,
Nag and Rakov [10] only found that 8 out of 52 positive ground flashes (15%) had detectable PBP
trains. Wu et al. [8], who analyzed 36 positive ground flashes from winter lightning measurement in
Japan found 26 were preceded by PBP (72%). The comparisons are given in Table 1. Comparing the
percentage from different locations, the values varied significantly. However, no dependence with
latitude were observed.
Table 1. Percentage of positive ground flashes (+CG) with detectable preliminary breakdown
pulses (PBP).
No of +CG
Researcher (Year)
Location
Type of Thunderstorms
(Breakdown of Sample Size)
Present study
Uppsala, Sweden
(59.837˝ N, 17.646˝ E)
Summer thunderstorm (51)
Qie et al. (2013)
Da Hinggan Ling, China
(50.4˝ N, 124.1˝ E)
Wu et al. (2013)
Sample
Size, N
PBP
Detected
PBP not
Detected
51
44 (86%)
7 (14%)
Summer thunderstorm (185)
185
89 (48%)
96 (52%)
Hokuriku, Japan
(~37˝ N, ~136˝ E)
Winter thunderstorm (36)
36
26 (72%)
10 (28%)
Nag and Rakov (2012)
Florida, US
(~29˝ N, ~82˝ W)
Summer thunderstorm (39)
Winter thunderstorm (13)
52
8 (15%)
44 (85%)
Schumann et al. (2013)
Sao Paulo, Brazil
(23.212˝ S, 45.867˝ W)
Summer thunderstorm (80)
80
80 (100%)
-
Note: Data listed in descending latitude.
Atmosphere 2016, 7, 39
6 of 18
Figure 1 shows the percentage of PBP observed according to distance, D of the positive ground
flashes from the measuring station. The majority of the PBP were detected when D were less than
25 km with
16 cases
This is followed by the 50 ď D < 100 km range with 11 cases6(26%)
and the
Atmosphere
2016,(38%).
7, 39
of 17
25 ď D < 50 km range with 10 cases (24%). For distances 100 km and above (assumed pure radiation),
≤ D < 50 km range with 10 cases (24%). For distances 100 km and above (assumed pure radiation),
there were25five
cases (12%) of PBP observed. In the sample data, there were two positive ground
there were five cases (12%) of PBP observed. In the sample data, there were two positive ground
flashes where
distance
werewere
not not
available.
highest
number
of was
PBPdetected
was detected
flashesthe
where
the distance
available. The
The highest
number
of PBP
when the when the
positive ground
were were
closest,
while
the
number
of PBP
detected
was
the positive
positiveflashes
ground flashes
closest,
while
thelowest
lowest number
of PBP
detected
was when
thewhen
positive
ground
flashes
were
furthest.
However,
for
distances
in
the
25–50
km
and
50–100
km
range,
we
did
ground flashes were furthest. However, for distances in the 25–50 km and 50–100 km range, we did
not see much difference since the number of PBP detected was almost equal.
not see much
difference since the number of PBP detected was almost equal.
PBP detected according to distance, D (Sample size, N = 42).
Figure Figure
1. PBP1. detected
according to distance, D (Sample size, N = 42).
3.2. Classification of PBP
3.2. Classification
of PBP
According to [36], preliminary breakdown is technically a cloud discharge since it takes place
inside the
signals the initiation
of a downward
leader and the
pulse burst
it produces
(having
According
tocloud.
[36], Itpreliminary
breakdown
is technically
a cloud
discharge
since
it takes place
a duration of the order of a millisecond) is the characteristic feature of the process. Since it is the first
inside the breakdown
cloud. It signals
the
initiation
of
a
downward
leader
and
the
pulse
burst
it
produces
(having
event that is associated with the ground flash, it is referred to as preliminary breakdown
a durationwhile
of the
order
of adischarges
millisecond)
is the
other
in-cloud
are called
IC. characteristic feature of the process. Since it is the first
In ourthat
analysis,
there were cases
several pulse
bursts,
a duration
of the order ofbreakdown
a
breakdown event
is associated
withwhere
the ground
flash,
it is having
referred
to as preliminary
millisecond,
observed are
preceding
first return stroke while in others, only a single pulse burst
while other
in-cloudwere
discharges
calledthe
IC.
could be seen. Based on this, the PBP can be categorized into single train PBP with 37 cases (84%) and
In ourmultiple
analysis,
there were cases where several pulse bursts, having a duration of the order of a
trains PBP with seven cases (16%). Table 2 gives the overview of the PBP classification while
millisecond,
were
observed
firstPBP
return
stroke while
in others,
only
a single pulse burst
Figure
2 shows
samplepreceding
waveforms the
for each
classification.
We analyzed
both the
characteristics
of
the
first
and
subsequent
PBP
trains
but
the
results
of
the
subsequent
PBP
trains
are
presented
could be seen. Based on this, the PBP can be categorized into single train PBP with 37
cases (84%) and
separately
in
Section
3.6.
multiple trains PBP with seven cases (16%). Table 2 gives the overview of the PBP classification while
Figure 2 shows sample
each
PBP
classification.
analyzed
Tablewaveforms
2. Classificationfor
of PBP
based
on the
number of trainsWe
preceding
the firstboth
RS. the characteristics
of the first and subsequent PBP trains but theICresults of the subsequent PBP trains are presented
Category 3.6.%
Observation
Activity
Observation
No of +CG
separately in Section
Duration
PBP followed by IC, then leader and RS
PBP followed by IC
5
ms
Table
2. Classification of PBP based10–60
on the
number
trainsbypreceding
RS.
PBPof
followed
IC, then RS the first 15
Single train
PBP (N = 37)
Category
Single
train PBP
(N = 37)
%
Multiple trains
PBP (N = 7)
84%
PBP not followed by IC
16%
PBP not followed by IC
PBP followed by IC
84%
PBP not followed by IC
Multiple
trains PBP
(N = 7)
-
IC Activity
Observation
PBP followed by IC Duration
10–120 ms
PBP followed by IC
-
10–60 ms
Note: Sample size, N = 44.PBP followed by IC, then RS
-
No of +CG
5
15
PBP followed by leader then RS
5
PBP followed by RS
12
10–120 ms
PBP followed by IC then RS
5
-
PBP followed by leader then RS
1
PBP followed by RS
1
16%
PBP not followed by IC
PBP followed by leader then RS
5
PBP followed by RS
12
Observation
PBP followed
by IC then RS
5
PBP followed by leader then RS
1
PBP followed
by IC,by
then
PBP followed
RS leader and RS 1
Note: Sample size, N = 44.
Atmosphere 2016, 7, 39
Atmosphere 2016, 7, 39
7 of 18
7 of 17
Figure2.2.Sample
Samplewaveforms
waveformsshowing
showingPBP
PBPclassifications
classifications (a)
(a)Single
Singletrain
trainPBP
PBPfollowed
followedby
byin-cloud
in-cloud
Figure
(IC)activity,
activity,then
thenleader
leaderand
andreturn-stroke
return-stroke(RS);
(RS);(b)
(b)Single
Singletrain
trainPBP
PBPfollowed
followedby
byIC
ICactivity
activitythen
thenRS;
RS;
(IC)
(c)Single
Singletrain
trainPBP
PBPfollowed
followedby
byleader
leaderthen
thenRS;
RS;(d)
(d)Single
Singletrain
trainPBP
PBPfollowed
followedby
byRS;
RS;(e)
(e)Multiple
Multipletrains
trains
(c)
PBPfollowed
followed by IC
trains
PBP
followed
by leader
thenthen
RS; (g)
trains
PBP
IC activity
activitythen
thenRS;
RS;(f)(f)Multiple
Multiple
trains
PBP
followed
by leader
RS;Multiple
(g) Multiple
PBP followed
by RS.by RS.
trains
PBP followed
Forsingle
singletrain
trainPBP
PBPcategory,
category,only
onlyone
onePBP
PBPtrain
trainwas
was observed
observed preceding
preceding the
the first
first return
return stroke.
stroke.
For
Most
of
these
PBP
trains
were
followed
by
IC
discharge
activity
having
a
duration
of
10–60
ms
Most of these PBP trains were followed by IC discharge activity having a duration of 10–60 ms
(20cases),
cases),while
whileothers
others(17
(17cases)
cases)did
didnot
nothave
haveany
anydetectable
detectableIC
ICactivity.
activity.For
Forthe
thePBP
PBPtrain
trainfollowed
followed
(20
by
IC
activity,
some
trains
were
followed
by
pronounced
leader
pulses
and
then
return
stroke
(5
by IC activity, some trains were followed by pronounced leader pulses and then return stroke (5
cases),
cases),
while
in
others
(15
cases),
they
were
followed
by
just
the
return
stroke
without
any
detectable
while in others (15 cases), they were followed by just the return stroke without any detectable leader
leader changes.
For PBP
train
with noICdetectable
five
of them
were followed
by
changes.
For PBP train
with no
detectable
activity, fiveICof activity,
them were
followed
by pronounced
leader
pronounced
leader
pulses
and
then
return
stroke,
while
the
remaining
12
were
followed
by
just
the
pulses and then return stroke, while the remaining 12 were followed by just the return stroke without
return
stroke without
any detectable leader pulses.
any
detectable
leader pulses.
For
the
case
of
multiple
trainsPBP,
PBP,we
weobserved
observedup
upto
tothree
threePBP
PBPtrains
trainspreceding
precedingthe
thefirst
firstreturn
return
For the case of multiple trains
stroke.
Since
there
were
no
other
ground
flashes
present
in
the
1
s
record,
we
assumed
that
the
stroke. Since there were no other ground flashes present in the 1 s record, we assumed that the multiple
multiple
belong
the same
positive
For five
this cases
category,
cases trains
of the
trains
PBPtrains
belongPBP
to the
sameto
positive
ground
flash.ground
For thisflash.
category,
of thefive
multiple
multiple
trains
PBP
were
followed
by
IC
having
a
duration
of
10–120
ms
and
then
followed
by a
PBP were followed by IC having a duration of 10–120 ms and then followed by a return stroke, while
return
stroke,
while
in
one
case,
the
PBP
trains
were
followed
by
pronounced
leader
pulses
and
then
in one case, the PBP trains were followed by pronounced leader pulses and then a return stroke and in
a returncase,
stroke
another
case,
the PBP
were
followed
by just the return stroke.
another
theand
PBPintrains
were
followed
bytrains
just the
return
stroke.
3.3.
3.3. Overall
Overall Pulse
Pulse Shape
Shape and
and Polarity
Polarity of
of the
the Initial
Initial Half
Half Cycle
Cycle
Typically,
Typically, pulses
pulses in
in the
the PBP
PBP train
train are
are bipolar.
bipolar. The
The majority
majority of
of the
the pulses
pulses have
have aa smooth
smooth initial
initial
rising
rising portion.
portion. Some
Some pulses
pulses have
have several
several small
small pulses
pulses superimposed
superimposed on
on the
the rising
rising part
part while
while aa few
few
others
have
multiple
peaks.
Based
on
the
pulses
initial
half
cycle
polarity,
three
types
of
PBP
train
others have multiple peaks. Based on the pulses initial half cycle polarity, three types of PBP train
Atmosphere 2016, 7, 39
8 of 18
have
have been
been identified.
identified. Out
Out of
of 44
44 positive
positive ground
ground flashes
flashes that
that have
have detectable
detectable PBP
PBP trains,
trains, 21
21 (48%)
(48%) have
have
the
same
polarity
as
the
first
return
stroke
(Type
1),
15
(34%)
have
the
opposite
polarity
the same polarity as the first return stroke (Type 1), 15 (34%) have the opposite polarity
(Type
2)
the
remaining
eight
(18%)
have
(Type
2) while
while
theShape
remaining
eight of
(18%)
have composite
composite
polarity (Type
(Type 3).
3). For
For Type
Type 3,
3, there
there are
are two
two
3.3.
Overall
Pulse
and Polarity
the Initial
Half Cycle polarity
distinct
distinct regions.
regions. The
The first
first region
region has
has either
either the
the same
same or
or opposite
opposite polarity
polarity to
to the
the return
return stroke
stroke but
but the
the
Typically,
pulses
in the PBP
train arefrom
bipolar.
Theregion.
majority
of the3–5
pulses
have
a smooth
initial
second
region
can
be
mixed
or
different
the
first
Figures
show
sample
waveforms
second region can be mixed or different from the first region. Figures 3–5 show sample waveforms
rising
portion.
Some
pulses have
several small pulses superimposed on the rising part while a few
for
for Type
Type 1,
1, Type
Type 2,
2, and
and Type
Type 33 respectively.
respectively.
others
have multiple peaks.
Basedmost
on the pulses
initial half
cycle polarity,
three types of
PBPfirst
train have
Previous
Previous studies
studies found
found that
that most PBP
PBP either
either have
have the
the same
same or
or opposite
opposite polarity
polarity to
to the
the first return
return
been
identified.
Out
of
44
positive
ground
flashes
that
have
detectable
PBP
trains,
21
(48%)
have
the
stroke
with
the
majority
having
the
same
polarity
as
the
first
return
stroke.
Ushio
et
al.
[4],
stroke with the majority having the same polarity as the first return stroke. Ushio et al. [4], for
for
same
polarity
as
the
first
return
stroke
(Type
1),
15
(34%)
have
the
opposite
polarity
(Type
2)
while
the
example,
example, found
found 17
17 out
out of
of 19
19 cases
cases (89.5%)
(89.5%) have
have the
the same
same polarity.
polarity. Nag
Nag and
and Rakov
Rakov [10]
[10] found
found that
that
remaining
eight
(18%) have
composite polarity
(Type 3). ForSchumann
Type 3, thereal.
are two
distinct regions.
The
seven
seven out
out of
of eight
eight cases
cases (87.5%)
(87.5%) have
have the
the same
same polarity.
polarity. Schumann et
et al. [7]
[7] found
found that
that 95%
95% of
of the
the
first
region
has
either
the
same
or
opposite
polarity
to
the
return
stroke
but
the
second
region
can
be
80
80 samples
samples have
have the
the same
same polarity
polarity as
as the
the first
first return
return stroke.
stroke. Wu
Wu et
et al.
al. [8],
[8], however,
however, found
found only
only 11
11
mixed
or
different
from
the
first
region.
Figures
3–5
show
sample
waveforms
for
Type
1,
Type
2,
and
(42%)
(42%) had
had the
the same
same polarity
polarity while
while the
the other
other 15
15 had
had opposite
opposite polarity.
polarity.
Type 3 respectively.
3.
(a)
Type
PBP—Same
polarity
PBP;
Zoom
of
PBP
having
the
same
Figure
polarity
PBP;
(b) (b)
Zoom
in ofin
having
the same
pulses
Figure 3.
3. (a)
(a)Type
Type1 11PBP—Same
PBP—Same
polarity
PBP;
(b)
Zoom
inPBP
of train
PBP train
train
having
the polarity
same polarity
polarity
pulses
as
the
positive
RS;
(c)
Zoom
in
of
the
subsequent
PBP
trains;
(d)
Zoom
in
of
positive
RS.
as
the
positive
RS;
(c)
Zoom
in
of
the
subsequent
PBP
trains;
(d)
Zoom
in
of
positive
RS.
pulses as the positive RS; (c) Zoom in of the subsequent PBP trains; (d) Zoom in of positive RS.
Figure
PBP—Opposite
Figure 4.
4. (a)
(a) Type
Type 22 PBP—Opposite
PBP—Opposite polarity
polarity PBP;
PBP; (b)
(b) Zoom
Zoom in
in of
of PBP
PBP train
train having
having opposite
opposite polarity
polarity
pulses
from
the
positive
RS;
(c)
Zoom
in
of
IC
activity;
(d)
Zoom
in
of
positive
RS.
(c)
Zoom
in
of
IC
activity;
(d)
Zoom
in
of
positive
RS.
pulses from the positive RS; (c) Zoom in of IC activity; (d) Zoom in of positive RS.
Atmosphere 2016, 7, 39
Atmosphere 2016, 7, 39
9 of 18
9 of 17
Figure
Type3 3PBP—Composite
PBP—Composite polarity
polarity PBP;
2 different
Figure
5. 5.(a)(a)Type
PBP; (b)
(b) Zoom
ZoomininofofPBP
PBPtrain
trainhaving
having
2 different
regions;
(c)
Zoom
in
of
the
first
region
showing
PBP
pulses
with
opposite
polarity
from
the
positive
regions; (c) Zoom in of the first region showing PBP pulses with opposite polarity from the
positive
RS;
(d)
Zoom
in
of
the
second
region
showing
pulses
with
mixed
polarity;
(e)
Zoom
in
of
IC
activity;
RS; (d) Zoom in of the second region showing pulses with mixed polarity; (e) Zoom in of IC activity;
Zoom
positiveRS.
RS.
(f) (f)
Zoom
in in
ofof
positive
Some studies also showed that the PBP can exhibit both polarities. Gomes and Cooray [5], in
Previous
studies found
that most
either
have
the polarity.
same or opposite
polarity
the
return
particular, identified
four types
of PBPPBP
based
on the
initial
They found
that 57to
out
offirst
71 cases
stroke
with
the
majority
having
the
same
polarity
as
the
first
return
stroke.
Ushio
et
al.
[4],
for
example,
(80%) had the same polarity, five had opposite polarity (7%), six exhibited both polarity (9%) while
found
17 outthree
of 19had
cases
(89.5%)
have the
same
polarity.
Nagetand
Rakov
foundseveral
that seven
the other
irregular
polarity
(4%).
Recently,
Zhang
al. [6]
also [10]
identified
typesout
of of
eight
cases
haveinthe
polarity.
et al.the
[7]same
found
that 95%
of the
samples
have
PBP.
They(87.5%)
found that
onesame
location,
55% Schumann
of the PBP had
polarity
as the
first80
return
stroke,
the39%
same
polarity
as thepolarity
first return
stroke.
Wu et al.polarity
[8], however,
only
11 (42%)
the81%
same
had
the opposite
and 6%
had composite
while infound
another
location,
theyhad
found
polarity
while
the
had opposite
of the PBP
had
theother
same15
polarity,
15% hadpolarity.
the opposite polarity and 4% had composite polarity. Qie et al.
[9]Some
also classified
the showed
PBP into that
threethe
types,
typeboth
S (same),
type DGomes
(different),
and type[5],
C in
studies also
PBP namely
can exhibit
polarities.
and Cooray
(chaotic)identified
accordingfour
to the
differences
in theon
initial
polarity
of bipolar
theout
first
particular,
types
of PBP based
the initial
polarity.
Theypulses
foundfrom
that 57
ofreturn
71 cases
stroke,
account
for 62.92%,
23.60%,
and 13.48%,
respectively.
The statistics
comparison
are
(80%)
hadwhich
the same
polarity,
five had
opposite
polarity
(7%), six exhibited
bothand
polarity
(9%) while
shown
in
Table
3.
the other three had irregular polarity (4%). Recently, Zhang et al. [6] also identified several types of
PBP. They found that in one location, 55% of the PBP had the same polarity as the first return stroke,
Table 3. Percentage of PBP according to initial half cycle polarity.
39% had the opposite polarity and 6% had composite polarity while in another location, they found
(Year)the same polarity,
Location 15% had
Sample
Size, N Type
*
Polarity
No composite
of PBP
%polarity.
81% of Researcher
the PBP had
the opposite
polarity
and 4% had
1
Same
21
48%
Qie et al. [9] also classified the
PBP
into
three
types,
namely
type
S
(same),
type
D
(different),
and
Uppsala, Sweden
Present study
44
2
Opposite
15
34%
(59.837°N,
17.646°E)
type C (chaotic) according to the differences in the initial polarity
pulses from
the first
3 of bipolar
Composite
8
18% return
stroke, which account for 62.92%,
23.60%,
The
statistics and
are
Same
17 comparison
89.5%
Hokuriku,
Japan and 13.48%, respectively.
Ushio et al. (1998)
19
(~37°N,
~136°E)
Opposite
2
10.5%
shown in Table 3.
Nag and Rakov (2012)
Florida, US
7
1
Same
76
80
Opposite
4
Sample Size, N
Type *
Polarity
No of PBP
+PBP
Same
11
36
1
Same
21
−PBP
Opposite
15
44
2
Opposite
15
a
Same
57
b 3 Composite
58
Composite
71
c Opposite
6
Same
17
d
Irregular
3
19
52
-
Same
~82°W)
Opposite
Table 3. (~29°N,
Percentage
of PBP according to initial- half cycle
polarity.
Schumann et al. (2013)
Researcher (Year)
Wu et al. (2013)
Present study
Gomes and Cooray
(2004)
Ushio et al. (1998)
Sao Paulo, Brazil
(23.212°S, 45.867°W)
Location
Hokuriku, Japan
(~37°N, ~136°E)
Uppsala, Sweden
(59.837˝ N, 17.646˝ E)
Uppsala, Sweden
(59.8°N, 17.6°E)
Hokuriku, Japan
(~37˝ N, ~136˝ E)
-
Opposite
2
87.5%
12.5%
95%
5%
%
42%
48%
58%
34%
80%
7%
18%
9%
89.5%
4%
10.5%
Atmosphere 2016, 7, 39
10 of 18
Table 3. Cont.
Researcher (Year)
Location
Sample Size, N
Nag and Rakov (2012)
Florida, US
(~29˝ N, ~82˝ W)
52
Schumann et al. (2013)
Sao Paulo, Brazil
(23.212˝ S, 45.867˝ W)
80
Wu et al. (2013)
Hokuriku, Japan
(~37˝ N, ~136˝ E)
36
Type *
+PBP
71
100
Zhang et al. (2013)
48
Da Hinggan Ling, China
(50.4˝ N, 124.1˝ E)
185
76
95%
4
5%
11
42%
Composite
5
7%
c
Opposite
6
9%
d
Irregular
3
4%
55
55%
39
39%
Same
Same
II
Opposite
III
Composite
Same
6
6%
39
81%
15%
II
Opposite
7
III
Composite
2
4%
56
62.9%
S
Qie et al. (2013)
87.5%
12.5%
58%
I
Guangzhou, China
(23.34˝ N, 113.36˝ E)
Same
Opposite
7
1
80%
I
Beijing, China
(39.94˝ N, 116.32˝ E)
Same
Opposite
%
57
b
Uppsala, Sweden
(59.8˝ N, 17.6˝ E)
Same
Opposite
No of PBP
15
´PBP
a
Gomes and Cooray (2004)
Polarity
Same
D
Opposite
21
23.6%
C
Chaotic
12
13.5%
* Type as defined by the authors.
3.4. PBP Peak Amplitude Relative to Return Stroke Peak
The ratio between the largest amplitude of PBP and the return stroke peak was also analyzed.
In most cases, the PBP peak values were much smaller than the first return stroke peak. On average,
the ratio of the PBP peak to the first return stroke peak is 0.26 for Type 1, 0.22 for Type 2, and 0.21 for
Type 3. This is in agreement with observation by previous researchers. Ushio et al. [4] found the mean
ratio to be 0.27 with values ranging between 0.02 and 1.9. On the other hand, both Zhang et al. [6]
and Wu et al. [8] found the mean values to be less than 0.20 while Qie et al. [9] found the mean ratio to
be 0.32, which was the highest mean ratio value among all studies compared. Based on the average
ratio of the PBP peak to the first return stroke peak, the PBP in positive ground flashes during summer
thunderstorms in Sweden can be considered as weak since the mean values for all three types are less
than half, ranging only between 0.21 and 0.26.
In one ground flash, the PBP was found to have a ratio higher than one (1.15), indicating that the
PBP peak was higher than the peak of the first return stroke. Ushio et al. [4] also found instances where
the peak ratio was higher than one (1.9) but Qie et al. [9] did not find any cases involving PBP peak
higher than the return stroke peak. For the one case where we observed the ratio to be higher than
one, the positive ground flash was quite close, about 23 km away from the measuring station and had
multiple subsequent return strokes. The initial polarity of the PBP was the same as the first return
stroke (Type 1).
The statistics and comparison with previous studies are given in Table 4. Comparing the mean
values from different locations, the ratio of the PBP peak to the first return stroke peak varied slightly,
ranging between 0.153 and 0.32. However, no dependence of the mean ratio with latitude can be seen.
Atmosphere 2016, 7, 39
11 of 18
Table 4. Comparison on PBP peak amplitude relative to return stroke peak with previous studies.
Researcher (Year)
Present study
Qie et al. (2013)
Location
Uppsala, Sweden
(59.837˝ N, 17.646˝ E)
Da Hinggan Ling, China
(50.4˝ N, 124.1˝ E)
Beijing, China
(39.94˝ N, 116.32˝ E)
Sample Size, N
44
185
100
Zhang et al. (2013)
Guangzhou, China
(23.34˝ N, 113.36˝ E)
Wu et al. (2013)
Ushio et al. (1998)
Hokuriku, Japan
(~37˝ N, ~136˝ E)
Hokuriku, Japan
(~37˝ N, ~136˝ E)
48
Polarity
PBP Peak Amplitude Relative to RS Peak
Max
Min
Arithmetic Mean
Same
1.15
0.03
0.26
Opposite
0.74
0.04
0.22
Composite
0.43
0.07
0.21
Same
0.99
0.03
0.32
Opposite
-
-
-
Chaotic
-
-
-
Same
0.524
0.021
0.197
0.186
Opposite
0.45
0.035
Composite
-
-
-
Same
0.594
0.043
0.195
Opposite
0.272
0.035
0.153
Composite
-
Same
36
Opposite
0.48
-
-
-
0.17
-
0.18
0.02
0.27
Same
19
Opposite
1.9
3.5. Temporal Characteristics
In order to analyze the temporal characteristics of the PBP, we determined the time interval
between the largest PBP and the first return stroke, pulse train duration, interpulse duration, individual
pulse duration, and total pulse duration for each type. The time interval between the largest PBP
and the first return stroke (PBP-RS separation) can be defined as the time between the peak of the
largest PBP and the peak of the first return stroke [5–10]. Pulse train duration can be defined as the
time interval between the peaks of the first and last pulses in the train [5–7]. Interpulse duration is
the time interval between the peaks of two consecutive pulses [5–9,14]. For individual pulse duration
and total pulse duration, we determined the pulse duration for the first half cycle (T1 ) and the second
half cycle (T2 ) of an individual bipolar pulse [5,7,9]. Thus, T1 + T2 is approximately equal to the total
pulse duration [5,7,9,14]. Figure 6 shows a sample waveform illustrating the temporal characteristics
of the PBP.
For Type 1, Type 2, and Type 3, the average PBP-RS separations were found to be 75.1 ms, 114.6 ms,
and 59.2 ms, while the pulse train durations were 5.9 ms, 2.6 ms, and 24.8 ms, respectively. Also, the
interpulse durations were found to be 115.1 µs, 93.7 µs, and 57.2 µs. As for individual pulse duration,
the average values for the first half cycle (T1 ) were 8.2 µs, 3.8 µs, and 4.9 µs while for the second half
cycle (T2 ) the values were 10.0 µs, 7.3 µs, and 8.3 µs. Total pulse durations (T1 + T2 ) were found to be
16.9 µs, 10.0 µs, and 12.8 µs. These values were for Type 1, Type 2, and Type 3 respectively.
The results were then compared with previous studies by Zhang et al. [6] and Gomes and
Cooray [5] since both studies reported similar types of PBP trains based on the pulses initial half cycle
polarity. We also compared our results with Wu et al. [8] since they separated their results based on the
polarity of the PBP. The statistics are given in Table 5.
temporal characteristics of the PBP.
For Type 1, Type 2, and Type 3, the average PBP-RS separations were found to be 75.1 ms,
114.6 ms, and 59.2 ms, while the pulse train durations were 5.9 ms, 2.6 ms, and 24.8 ms, respectively.
Also, the interpulse durations were found to be 115.1 µ s, 93.7 µ s, and 57.2 µ s. As for individual pulse
duration, the average values for the first half cycle (T1) were 8.2 µ s, 3.8 µ s, and 4.9 µ s while for the
Atmosphere 2016, 7, 39
12 of 18
second half cycle (T2) the values were 10.0 µs, 7.3 µs, and 8.3 µs. Total pulse durations (T1 + T2) were found
to be 16.9 µs, 10.0 µs, and 12.8 µs. These values were for Type 1, Type 2, and Type 3 respectively.
(a)
(b)
(c)
Figure6.6.Temporal
Temporalcharacteristics
characteristicsofofthe
thePBP
PBP(a)
(a)Sample
Samplewaveform
waveformshowing
showingPBP-RS
PBP-RSseparation;
separation;
Figure
(b)Zoom
Zoomininofofthe
thewaveform
waveformshowing
showingthe
thepulse
pulsetrain
trainduration;
duration;(c)
(c)Zoom
Zoomininofofthe
thewaveform
waveformshowing
showing
(b)
the
interpulse
duration,
pulse
duration
for
the
first
half
cycle
(T
1) and the second half cycle (T2) of an
the interpulse duration, pulse duration for the first half cycle (T1 ) and the second half cycle (T2 ) of an
individualbipolar
bipolarpulse,
pulse,and
andtotal
totalpulse
pulseduration
duration(T(T1++TT2).).
individual
1
2
The results were then compared with previous studies by Zhang et al. [6] and Gomes and Cooray [5]
since both studies
reported
similaroftypes
of PBP trains
based onwith
theprevious
pulses initial
half cycle polarity.
Table
5. Comparison
PBP temporal
characteristics
studies.
Researcher
(Year)
Present
study
Gomes
and
Cooray
(2004)
Location
Uppsala,
Sweden
(59.837˝ N,
17.646˝ E)
Uppsala,
Sweden
(59.8˝ N, 17.6˝ E)
Sample
Size,
N
44
71
Polarity
PBP-RS
Separation
(ms)
Individual Pulse
Duration (µs)
Pulse Train
Duration
(ms)
T1
T2
T1 + T2
Interpulse
Duration
(µs)
Same
75.1
5.9
8.2
10.0
16.9
Opposite
114.6
2.6
3.8
7.3
10.0
9.7
Composite
59.2
24.8
4.9
8.3
12.8
57.2
Same
56
3
19
19
38
96
Reg. 1
81
1.3
13
13
27
62
Reg. 2
77
2.8
12
14
26
38
Opposite
44
2.1
16
15
31
51
Irregular
-
-
-
-
-
-
Composite
115.1
Atmosphere 2016, 7, 39
13 of 18
Table 5. Cont.
Researcher
(Year)
Location
Beijing, China
(39.94˝ N,
116.32˝ E)
Zhang
et al. (2013)
Guangzhou,
China (23.34˝ N,
113.36˝ E)
Wu et al.
(2013)
Hokuriku,
Japan (~37˝ N,
~136˝ E)
Sample
Size,
N
100
48
PBP-RS
Separation
(ms)
Polarity
Pulse Train
Duration
(ms)
Individual Pulse
Duration (µs)
T1 + T2
Interpulse
Duration
(µs)
T1
T2
Same
94.2
3.1
-
-
21
141
Opposite
112.7
2.3
-
-
23
100
Composite
-
-
-
-
-
-
Same
99.5
5.1
-
-
31
256
Opposite
112.1
2.3
-
-
26
195
Composite
-
-
-
-
-
-
0.8
-
-
-
44
-
-
-
47
Same
36
17
1.1
Opposite
Note: Reg. = Region.
In general, our values were comparable to that of Zhang et al. [6] who studied PBP trains at two
different locations in China. Their values for both PBP-RS separation and pulse train duration for PBP
having opposite polarity to the return stroke were similar to those which we obtained. Our PBP-RS
separations for PBP with the same polarity were slightly lower while our pulse train durations were
slightly higher. Our interpulse duration and total pulse duration were lower for both Type 1 and 2.
Results from Gomes and Cooray [5], on the other hand, were slightly different despite being from the
same location. Our PBP-RS separation, pulse train duration, and interpulse duration were higher while
individual pulse duration and total pulse duration were lower, for all PBP types. One possible reason
for the difference could be because we had a smaller sample size and this could affect the results to a
certain extent. As for the findings from Wu et al. [8], their pulse train duration and interpulse duration
were smaller than the rest of the results compared here.
3.6. Characteristics of Subsequent PBP Trains
Out of seven positive ground flashes observed to have multiple trains PBP, six flashes had one
subsequent PBP train while one flash had two subsequent PBP trains following the first PBP train.
Similar to the first PBP trains, the pulse structure for the subsequent PBP trains is typically bipolar.
Some pulses have smooth initial rising portion while others have several small pulses superimposed on
the rising part. A few pulses have multiple peaks. Based on the pulses initial half cycle, polarity of two
PBP trains were found to be opposite to that of the first return stroke. Five PBP trains had composite
polarity in a single region while the remaining one had composite polarity with two distinct regions.
Characteristics of the eight subsequent PBP trains are given in Table 6. Compared to the first PBP
train, the parameter values obtained for the subsequent PBP trains were relatively smaller. The PBP
peak amplitude relative to the return stroke peak, for example, was found to be 0.14 on average while
the value for the first PBP train was 0.24. The pulse train duration, individual pulse duration, and
interpulse duration for the subsequent PBP trains were found to be 3.63 ms, 11.05 µs, and 43.78 µs
respectively compared to 7.41 ms, 13.80 µs, and 95.48 µs of the first PBP train. Additionally, the
time interval between the largest PBP and the first return stroke (PBP-RS separation) was found to
be 73.23 ms.
Table 6. Comparison on PBP characteristics between first and subsequent PBP trains.
Researcher
(Year)
Present
study
PBP
Category
Sample
Size, N
PBP Peak Amplitude
Relative to RS Peak
Max
Min
PBP-RS
Pulse
Separation Train
Duration
Arithmetic (ms)
(ms)
Mean
Individual Pulse
Duration (µs)
T1
T2
T1 + T2
Interpulse
Duration
(µs)
First PBP
train
44
1.15
0.03
0.24
85.66
7.41
6.08
8.80
13.80
95.48
Subsequent
PBP trains
8
0.38
0.04
0.14
73.23
3.63
5.02
7.68
11.05
43.78
Atmosphere 2016, 7, 39
14 of 18
4. Discussion
4.1. Reasons for No Detection of PBP
There could be several reasons why some PBP were not detected in positive ground flashes. One
is that the PBP amplitude might be relatively small, below noise level of the measurement site, making
it too small to be detected. In a recent study for negative ground flashes, Marshal et al. [17] said that
the detection of PBP before negative ground flashes regardless of amplitude requires the sensor to be
close enough to the parent CG flash to reveal pulses above the local noise level and the sensor’s noise
level. Even though their study was restricted only for negative ground flashes (due to lack of statistics
on positive ground flashes), our observation seems to agree with their suggestion since we found
that most of the PBP were detected when the positive ground flashes were close (less than 25 km).
Also, all our seven positive ground flashes without PBP detected were quite far at distances ranging
from 64–126 km.
The pre-trigger setting could also play a role. Electric field measurements by Rust et al. [37]
showed that the intracloud discharge activity can last on average 241 ms (between 40–800 ms) while
Fuquay [35] indicated that it can last on average 130 ms (between 65–210 ms). Based on this, it
is possible that some of the PBP might not have been detected because they occurred outside the
pre-trigger setting (i.e., 200 ms) of our transient recorder.
4.2. Different Polarity of PBP in Positive Ground Flashes
The basic charge structure of a normal thundercloud is a vertical tripole consisting of three regions:
the main positive charge centre at the top, the main negative charge centre in the middle and a smaller
positive charge pocket at the lower level [31]. For thunderstorms with normal tripolar structure, the
same polarity PBP (Type 1) can be viewed as the interaction between the main positive and main
negative charge centres while the opposite polarity PBP (Type 2) can be viewed as the interaction
between the main negative charge centre and the lower positive cloud region.
For such a case, the PBP-RS separation for same polarity PBP (Type 1) should probably be higher
than the opposite polarity PBP (Type 2) since the main positive charge centre is located at higher
altitude compared to the main negative charge centre. The PBP/RS ratio (representing the intensity
of the PBP) between Type 1 and Type 2 should probably also be noticeably different considering one
PBP type occurs further away from the measuring antenna. However, our findings did not support
this hypothesis. Our results showed that the PBP-RS separation for Type 2 is significantly higher
than Type 1 and the PBP/RS ratio between the two PBP types is almost similar. The observation of
Wu et al. [8] also did not support this hypothesis since their PBP-RS separations for both +PBP and
´PBP are almost the same, and they speculated that the difference in initial polarities of +PBP and
´PBP in positive ground flashes is caused by different directions of channel propagation.
According to Nag and Rakov [10], the basic tripolar structure of the thundercloud is not conducive
to the production of positive lightning while [31] stated that the tripolar structure is more consistent
with intracloud discharges, negative ground flashes, and air discharges, which are the dominant
lightning types in isolated thunderclouds. In their study, Nag and Rakov [10] discussed six conceptual
cloud charge configurations and scenarios that have been observed or hypothesized over the years and
lead to the production of positive lightning. The cloud configurations are the tilted dipole, positive
monopole, inverted dipole, unusually large lower positive charge region, negative in-cloud leader
channel cut-off, and branching of in-cloud channel.
For the case of inverted dipole, the main positive and main negative charge regions were
found at altitudes where, respectively, main negative and main positive charge would be found
in normal-polarity storms [38]. In addition, previous researchers also found that there is a smaller
negative charge region near the bottom of the cloud that serves to facilitate the positive ground flashes
(as discussed by [10] and references therein). Same polarity PBP (Type 1) can be due to interaction
between the main positive charge centre in the middle and the lower negative charge region while
Atmosphere 2016, 7, 39
15 of 18
opposite polarity PBP (Type 2) can be due to interaction between the main negative charge centre at
the top and the main positive charge centre. Considering that the inverted dipole has been suggested
as the dominant charge structure during the dissipating stage of ordinary thunderstorms [39], and
the presence of smaller negative charge region at the lower level that can trigger same polarity PBP
(Type 1), this might explain why our percentage of same polarity PBP (Type 1) is higher than opposite
polarity PBP (Type 2). The inverted dipole charge configuration would also explain why we have
larger PBP-RS for opposite polarity PBP (Type 2) because the main negative centre now is located
above the main positive centre.
According to Cooray and Scuka [29], opposite polarity PBP can take place between the main
negative charge centre and the lower positive charge centre, similar to PBP in negative ground flashes
but for the case of positive ground flashes, most of the charges in the negative charge centre are
neutralized in neutralizing the lower positive charge. As a result, the vertical charge channel created
is repolarized in the field of the main positive charge centre and serves to launch a positive leader
towards the ground [6]. This would result in positive ground flashes with opposite polarity PBP.
For composite polarity PBP (Type 3), Gomes and Cooray [5] suggested that it may be viewed as
the interaction between charge centres when the discharge channel is not vertical while Qie et al. [9]
suggested that PBP trains with composite polarity could be due to breakdown discharges between
both upper and lower charge layers. According to [40], the bi-directional, zero-net-charge leader
concept of lightning development has become the major key in interpreting various physical processes
in lightning. The essence of this concept, according to [40] is: lightning initiation in the electrified
cloud occurs as a bi-directional, bi-polar, zero-net-charge leader and electrodeless discharge. In the
bi-directional leader concept discussed in [3], a negatively charged channel initiated from the outer
boundary of the negative charge centre, propagates downwards while the positively charged channel
penetrates the negative charge centre. The same should also be true for the positive leader, initiated
from the outer boundary of the positive charge centre, the positively charged channel propagates
downwards while the negatively charged channel penetrates the positive charge centre. Electric field
changes recorded during this bi-direction movement of the charges would result in the same polarity
pulses when the channel is vertical. However, it was observed by previous researchers that the channel
can be curved and not straight (as discussed by [5] and references therein). In cases where the channel
is not vertical, pulses in the waveform would have mixed polarity, and this, we believe, is the reason
for the composite polarity PBP (Type 3).
5. Summary of Results
Characteristics of the PBP in 51 positive ground flashes, based on remote electric field
measurement during 2014 summer thunderstorms in Sweden have been presented and the results are
summarized as follows:
(i)
(ii)
(iii)
(iv)
(v)
PBP trains were observed in 44 (86%) of the cases while for the remaining seven (14%) no PBP
trains were detected.
From the 44 cases, multiple trains PBP were observed in seven (16%) of the cases while 37 (84%)
were single train PBP.
Based on the initial polarity of the pulses, we identified three types of PBP. Of the PBP 48% have
the same polarity as the first return stroke (Type 1), 34% have the opposite polarity (Type 2) while
the remaining 18% have composite polarity (Type 3).
Pulses in the PBP train are typically bipolar with the majority having a smooth initial rising
portion. Some have several small pulses superimposed on the rising part while a few others have
multiple peaks. The average ratios of the PBP peak to the first return stroke lie between 0.21 and
0.26. One case of PBP, however, registered a peak value higher than the first return stroke.
For Type 1, Type 2, and Type 3, the PBP-RS separations were found to be 75.1 ms, 114.6 ms, and
59.2 ms while the pulse train durations were 5.9 ms, 2.6 ms, and 24.8 ms, respectively. Also,
Atmosphere 2016, 7, 39
(vi)
(vii)
16 of 18
the interpulse durations were found to be 115.1 µs, 93.7 µs, and 57.2 µs. For individual pulse
duration, the average values for the first half cycle (T1 ) were 8.2 µs, 3.8 µs, and 4.9 µs while
for the second half cycle (T2 ) the values were 10.0 µs, 7.3 µs, and 8.3 µs. Total pulse durations
(T1 + T2 ) were found to be 16.9 µs, 10.0 µs, and 12.8 µs. These values were for Type 1, Type 2, and
Type 3 respectively.
For multiple trains PBP (N = 7), six flashes have one subsequent PBP train while one flash has
two subsequent PBP trains following the first PBP train.
Compared to the first PBP train, characteristics of the subsequent PBP trains (N = 8) were
relatively smaller. The PBP peak amplitude relative to the return stroke peak, pulse train duration,
individual pulse duration, and interpulse duration for the subsequent PBP trains were found to
be 0.14, 3.63 ms, 11.05 µs, and 43.78 µs respectively. Additionally, the time interval between the
largest PBP and the first return stroke (PBP-RS separation) was found to be 73.23 ms.
6. Conclusions
Based on the findings, we have come to several conclusions. First, the non-detection of PBP in
some of the positive ground flashes could be due to a relatively small PBP amplitude below noise
level and their occurrence outside the pre-trigger setting of the transient recorder. Second, the PBP in
positive ground flashes during summer thunderstorms in Sweden are weak since the average ratios of
the PBP peak to the first return stroke peak lie only between 0.21 and 0.26. We also discussed possible
reasons for the different types of PBP observed. Based on the conceptual charge cloud configuration
discussed by Nag and Rakov [10], we found that the inverted dipole is consistent with our observation.
The same polarity PBP (Type 1) could be due to the interaction between the main positive charge centre
in the middle and the lower negative charge region while opposite polarity PBP (Type 2) could be due
to the interaction between the main negative charge centre at the top and the main positive charge
centre. We also believe that composite polarity PBP (Type 3) is due to the bi-directional leader process
in a non-vertical channel.
Acknowledgments: The authors would like to express their gratitude to all the people who have directly or
indirectly contribute towards the successful completion of this paper. Participation of Dalina Johari is funded
by the Ministry of Education of Malaysia and Universiti Teknologi Mara Malaysia. Participations of Vernon
Cooray and Mahbubur Rahman are funded by the fund from B. John F. and Svea Andersson donation at Uppsala
University. Participation of Mohd Muzafar Ismail is funded by the Ministry of Education of Malaysia and
Universiti Teknikal Malaysia Melaka. We would also like to thank Shri Ram Sharma from Tribhuvan University
Kathmandu, Nepal for a fruitful discussion during his stay at Uppsala University. Also many thanks to Thomas
Götschl for the assistance in the measurement setup and acquisition of lightning data from the Swedish LLS
database. Finally, the authors would like to acknowledge the Division of Electricity, Ångström Laboratory, Uppsala
University, for the excellent facility provided to carry out this research.
Author Contributions: The study was completed with cooperation from all authors. Dalina Johari as the first
author prepared and carried out the measurements, collected the data, analyzed the data, and wrote the manuscript.
Vernon Cooray had the idea and checked the validation of the measurements analysis. Mahbubur Rahman
reviewed the manuscript. Pasan Hettiarachchi and Mohd Muzafar Ismail supported measurement technique and
analysis. All authors agreed with the submission of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
Rakov, V.A.; Uman, M.A. Positive and bipolar lightning discharges to ground. In Lightning: Physics and
Effects; Cambridge University Press: Cambridge, UK, 2003; pp. 214–240.
Rakov, V.A.; Uman, M.A. Downward negative lightning discharges to ground. In Lightning: Physics and
Effects; Cambridge University Press: Cambridge, UK, 2003; pp. 108–213.
Cooray, V. Mechanism of the lightning flash. In The Lightning Flash; Cooray, V., Ed.; The Institution of
Engineering and Technology: London, UK, 2014; pp. 119–229.
Ushio, T.; Kawasaki, Z.-I.; Matsu-ura, K.; Wang, D. Electric fields of initial breakdown in positive ground
flash. J. Geophys. Res. 1998, 103, 14135–14139. [CrossRef]
Atmosphere 2016, 7, 39
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
17 of 18
Gomes, C.; Cooray, V. Radiation field pulses associated with the initiation of positive cloud to ground
lightning flashes. J. Atmos. Sol. Terr. Phys. 2004, 66, 1047–1055. [CrossRef]
Zhang, Y.; Zhang, Y.; Lu, W.; Zheng, D. Analysis and comparison of initial breakdown pulses for positive
cloud-to-ground flashes observed in Beijing and Guangzhou. Atmos. Res. 2013, 129–130, 34–41. [CrossRef]
Schumann, C.; Saba, M.M.F.; da Silva, R.B.G.; Schulz, W. Electric fields changes produced by positives
cloud-to-ground lightning flashes. J. Atmos. Sol. Terr. Phys. 2013, 92, 37–42. [CrossRef]
Wu, T.; Takayanagi, Y.; Funaki, T.; Yoshida, S.; Ushio, T.; Kawasaki, Z.-I.; Morimoto, T.; Shimizu, M.
Preliminary breakdown pulses of cloud-to-ground lightning in winter thunderstorms in Japan. J. Atmos. Sol.
Terr. Phys. 2013, 102, 91–98. [CrossRef]
Qie, X.; Wang, Z.; Wang, D.; Liu, M. Characteristics of positive cloud-to-ground lightning in da Hinggan Ling
forest region at relatively high latitude, northeastern China. J. Geophys. Res. Atmos. 2013, 118, 13393–13404.
[CrossRef]
Nag, A.; Rakov, V.A. Positive lightning: An overview, new observations, and inferences. J. Geophys. Res.
2012, 117. [CrossRef]
Clarence, N.D.; Malan, D.J. Preliminary discharge processes in lightning flashes to ground. Q. J. R. Meteorol.
Soc. 1957, 83, 161–172. [CrossRef]
Brook, M. Breakdown electric fields in winter storms. Res. Lett. Atmos. Electr. 1992, 12, 47–52.
Gomes, C.; Cooray, V.; Jayaratne, C. Comparison of preliminary breakdown pulses observed in Sweden and
in Sri Lanka. J. Atmos. Sol. Terr. Phys. 1998, 60, 975–979. [CrossRef]
Nag, A.; Rakov, V.A. Electric field pulse trains occurring prior to the first stroke in negative cloud-to-ground
lightning. IEEE Trans. Electromagn. Compat. 2009, 51, 147–150. [CrossRef]
Baharudin, Z.A.; Ahmad, N.A.; Fernando, M.; Cooray, V.; Mäkelä, J.S. Comparative study on preliminary
breakdown pulse trains observed in Johor, Malaysia and Florida, USA. Atmos. Res. 2012, 117, 111–121.
[CrossRef]
Baharudin, Z.A.; Fernando, M.; Ahmad, N.A.; Mäkelä, J.S.; Rahman, M.; Cooray, V. Electric field changes
generated by the preliminary breakdown for the negative cloud-to-ground lightning flashes in Malaysia and
Sweden. J. Atmos. Sol. Terr. Phys. 2012, 84–85, 15–24. [CrossRef]
Marshall, T.; Schulz, W.; Karunarathna, N.; Karunarathne, S.; Stolzenburg, M.; Vergeiner, C.; Warner, T. On
the percentage of lightning flashes that begin with initial breakdown pulses. J. Geophys. Res. Atmos. 2014,
119, 445–460. [CrossRef]
Weidman, C.D.; Krider, E.P. The radiation field wave forms produced by intracloud lightning discharge
processes. J. Geophys. Res. 1979, 84, 3159–3164. [CrossRef]
Bils, J.R.; Thomson, E.M.; Uman, A. Electric field pulses in close lightning cloud flashes. J. Geophys. Res. 1988,
93, 15933–15940. [CrossRef]
Villanueva, Y.; Rakov, V.A.; Uman, M.A.; Brook, M. Microsecond-scale electric field pulses in cloud lightning
discharges. J. Geophys. Res. 1994, 99, 14353–14360. [CrossRef]
Sharma, S.R.; Fernando, M.; Gomes, C. Signatures of electric field pulses generated by cloud flashes. J. Atmos.
Sol. Terr. Phys. 2005, 67, 413–422. [CrossRef]
Stolzenburg, M.; Marshall, T.C.; Karunarathne, S.; Karunarathna, N.; Vickers, L.E.; Warner, T.A.; Orville, R.E.;
Betz, H.D. Luminosity of initial breakdown in lightning. J. Geophys. Res. Atmos. 2013, 118, 2918–2937.
[CrossRef]
Stolzenburg, M.; Marshall, T.C.; Karunarathne, S.; Karunarathna, N.; Orville, R.E. Leader observations
during the initial breakdown stage of a lightning flash. J. Geophys. Res. Atmos. 2014, 119, 12198–12221.
[CrossRef]
Campos, L.Z.S.; Saba, M.M.F. Visible channel development during the initial breakdown of a natural negative
cloud-to-ground flash. Geophys. Res. Lett. 2013, 40, 4756–4761. [CrossRef]
Karunarathne, S.; Marshall, T.C.; Stolzenburg, M.; Karunarathna, N.; Vickers, L.E.; Warner, T.A.; Orville, R.E.
Locating initial breakdown pulses using electric field change network. J. Geophys. Res. Atmos. 2013, 118,
7129–7141. [CrossRef]
Bitzer, P.M.; Christian, H.J.; Stewart, M.; Burchfield, J.; Podgorny, S.; Corredor, D.; Hall, J.; Kuznetsov, E.;
Franklin, V. Characterization and applications of VLF/LF source locations from lightning using the
Huntsville Alabama Marx Meter Array. J. Geophys. Res. Atmos. 2013, 118, 3120–3138. [CrossRef]
Atmosphere 2016, 7, 39
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
18 of 18
Wu, T.; Yoshida, S.; Akiyama, Y.; Stock, M.; Ushio, T.; Kawasaki, Z. Preliminary breakdown of intracloud
lightning: Initiation altitude, propagation speed, pulse train characteristics, and step length estimation.
J. Geophys. Res. Atmos. 2015, 120, 9071–9086. [CrossRef]
Nag, A.; Rakov, V.A. Pulse trains that are characteristic of preliminary breakdown in cloud-to-ground
lightning but are not followed by return stroke pulses. J. Geophys. Res. Atmos. 2008, 113. [CrossRef]
Cooray, V.; Scuka, V. What attracts a lightning flash to ground? In Proceedings of the 10th International
Conference on Atmospheric Electricity, Osaka, Japan, 10–14 June 1996; pp. 256–259.
Levine, D.M. Sources of the strongest RF radiation from lightning. J. Geophys. Res. 1980, 85, 4091–4095.
[CrossRef]
Williams, E. Charge structure and geographical variation of thunderclouds. In The Lightning Flash; Cooray, V.,
Ed.; The Institution of Engineering and Technology: London, UK, 2014; pp. 1–14.
Galvan, A.; Fernando, M. Operative Characteristics of a Parallel-Plate Antenna to Measure Vertical Electric Fields
from Lightning Fields from Lightning Flashes; Report UURIE 285-00; Uppsala University: Uppsala, Sweden,
2000.
Sonnadara, U.; Kathriarachchi, V.; Cooray, V.; Montano, R.; Götschl, T. Performance of lightning locating
systems in extracting lightning flash characteristics. J. Atmos. Sol. Terr. Phys. 2014, 112, 31–37. [CrossRef]
Sonnadara, U.; Cooray, V.; Götschl, T. Characteristics of cloud-to-ground lightning flashes over Sweden.
Phys. Scr. 2006, 74, 541–548. [CrossRef]
Fuquay, D.M. Positive cloud-to-ground lightning in summer thunderstorms. J. Geophys. Res. Ocean. 1982, 87,
7131–7140. [CrossRef]
Cooray, V. Electromagnetic Fields of Lightning Flashes. In An Introduction to Lightning; Springer: Dordrecht,
The Netherlands, 2015; pp. 135–165.
Rust, W.D.; MacGorman, D.R.; Arnold, R.T. Positive cloud to ground lightning flashes in severe storms.
Geophys. Res. Lett. 1981, 8, 791–794. [CrossRef]
Rust, W.D.; MacGorman, D.R.; Bruning, E.C.; Weiss, S.A.; Krehbiel, P.R.; Thomas, R.J.; Rison, W.; Hamlin, T.;
Harlin, J. Inverted-polarity electrical structures in thunderstorms in the Severe Thunderstorm Electrification
and Precipitation Study (STEPS). Atmos. Res. 2005, 76, 247–271. [CrossRef]
Williams, E.; Yair, Y. The microphysical and electrical properties of sprite-producing thunderstorms. 2006,
225, 57–83.
Mazur, V. Physical processes during development of lightning flashes. C. R. Phys. 2002, 3, 1393–1409.
[CrossRef]
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons by Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).